2.1. Introduction
For patients who have lost many adjacent teeth, dental prostheses are widely used. These prostheses may be either removable full or partial dentures or fixed full or partial dentures. A common type of fixed denture is the “hybrid prosthesis” a term generally used to describe a wide variety of dental prostheses with materials of different composition. Specifically, denture teeth may be porcelain, plastic, or composite; denture may be acrylic or metal; and the denture may be fixed to a metal framework which is screwed into the jaw.
The term “hybrid prostheses” includes: the Branemark hybrid prosthesis; a fixed detachable prosthesis; a fixed hybrid prosthesis; a fixed-detachable implant-supported prosthesis; implant hybrid overdenture; and the hybrid prosthesis. These hybrid prostheses have a high rate of mechanical complications. In particular, based on a literature survey, Goodacre et al. reported the following complication rates: overdenture loss of retention/adjustment, 30%; esthetic veneer fracture (resin), 22%; overdenture relines, 19%; overdenture clip/attachment fracture, 17%; and esthetic veneer fracture (porcelain), 14%. Goodacre et al., 2003 J Prosthetic Dent 90(2) 121-132, “Clinical Complications with implants and Implant Prostheses.” Basically, whether resin or porcelain, the teeth are breaking off. Other mechanical complications include: the fracture of the resin support; fracture of attachment to underlying metal framework; or the fracture of the metal framework/implant bar.
The reasons for these mechanical complications are many. Specific reasons for failure include the fact that the classical hybrid framework was a flat platform to support denture teeth. They were not designed with individualized metal supports for denture teeth. There also was almost no vertical support component on the lingual to prevent fracture of teeth distal to the last implant. Taylor and Bergman, Laboratory Techniques for the Branemark System 1990, Quintessence. In some designs, the heat processed acrylic was retained by acrylic beads which in many cases were covered with an opaque agent that may contribute to the failure. In some cases, the acrylic was wrapped around the implant bar for additional retention.
Another prosthesis design was described by Beumer and Lewis in 1989, Beumer and Lewis, The Branemark Implant System, Clinical and Laboratory Procedures, 1989; Ishiyaku, EuroAmerical, Inc.; Lewis et al. 1989, Int J Oral Maxillofac Implants 4(2): 147-52 “The restoration of improperly inclined osseointegrated implants.” Their design actually gave individual support for the denture teeth but the rods were not parallel with each other. Furthermore, the teeth were added in sections with silicone matrices which are difficult to work with. In particular, it is difficult to get the denture teeth to maintain their position in the silicone matrices. Like the classical hybrid framework above, the vertical support component to the framework was insufficient. Additional vertical support would allow for greater structural durability. In this system, the lingual area was purposely left open to allow flasking like a partial denture as the heat processed acrylic resin needs a way to flow in. Another issue with this technique is that some of the teeth may change position in the flask once the wax is eliminated.
A number of ways to reduce the likelihood of fracture have been considered by the dental community. These ideas include: increasing the thickness of the veneer materials; the thickness of the acrylic denture teeth; and the thickness of the heat-cured denture resins; changing the framework such as the type of metal or fabrication; adding retentive elements to the framework; increasing the thickness of the framework; or mechanical chemical treatments of the framework.
Another complication with making improved hybrid frameworks is that each tooth must be individually drilled and individually fitted onto a post supporting the tooth in the denture material. This requires great precision and is extremely time consuming.
In summary, there is a long-felt need for techniques to make improved fixed or removable dentures or overdentures for a plurality of teeth.